This invention relates to a variable magnification optical system for a copying machine comprising a lens for projecting an image of an original document with a pre-selected magnification onto an imaging member. The invention further relates to a copying machine incorporating such an optical system.
In a scanning optical system the imaging member is moved in the direction of scan and the image of the document is projected a strip at a time onto the moving imaging member to reproduce the whole of the document as an image thereon. By varying the scan speed relative to the speed of the imaging member it is possible to alter the size of the image along the length of the belt, i.e. in the scanning direction. In full size copying, that is to say at unity magnification, the scan speed is equal to the speed of the imaging member. Increasing the scan speed makes the image shorter, i.e. reduction; and decreasing the scan speed makes the image longer, i.e. enlargement. To this end, a reference signal may be generated, the value of which is determined by the speed of the imaging member and the nominal pre-selected magnification. A second signal is generated indicative of the scan speed. The two signals are compared and the scan speed is varied until the second signal matches the reference signal. In practice, however, the actual magnification achieved may differ from the nominal magnification due to mechanical tolerances in the system.
The image size can also be varied in the direction transverse to the scan direction by moving the lens along its optical axis. The lens has a home position corresponding to nominal unity magnification, but again, due to tolerances in the system, this home position for the lens may yield an actual magnification which is at variance with the nominal magnification.
The concept of reducing or enlarging an image via a velocity mismatch of scanner image to a moving medium is known in the photographic and copy art. U.S. Pat. Nos. 3,126,809; 4,111,551 and 3,967,898 associated with the photographic art, disclose the production of copies from a negative, elongating or compressing the copy in a single direction. In U.S. Pat. No. 4,111,551 a negative and a photosensitive film are separately transported at different linear speed-past an elongated pair of drive rollers. The extent of image compression or elongation can be varied continually by means of a differential speed drive mechanism interconnecting the drive roller. In U.S. Pat. No. 3,126,809 a negative and print paper are moved relative to each other while the negative is being illuminated through a moving mode with a light slit. In U.S. Pat. No. 3,967,898 a predetermined amount of distortion in one direction is produced by varying the relative motions of a film image sheet and a superimposed photosensitive material sheet across a light slit.
A scanning system is also disclosed in U.S. Pat. No. 3,861,797 in which an original is reproduced in one direction at the original size and then elongated or compressed in a second direction. An original document is placed on a movable platen and illuminated from above by a light source so as to cause an image to be projected by a lens beneath the platen through a pair of slits and a sensitive film disposed in an image plane. The relative speed of the plate and the film are varied to obtain the desired unidirectional magnification.
U.S. Pat. No. 4,543,643 discloses a system for setting the magnification in a copying machine, which comprises means for manually inputting a numerical value, a memory means having a plurality of memory locations, selection key means for selecting one from the plurality of memory locations, and control means for storing a numerical value input by the numerical value input means in said one of the memory locations selected by the selection key means, so that the copying apparatus operates to set copying magnification by a numerical value stored in said one of the memory locations.
Japanese Patent No. 60-151662 discloses a memory means in a copying machine using values stored in a ROM and RAM memory locations as numerical copying magnification data. This data can be automatically selected and displayed when the operator demands magnification display.
Japanese Patent No. 58-54358 teaches a copying machine including means to change the magnification of an original to selected magnifications on the basis of size relation, in one direction or the other, between the original and copy.
U.S. Pat. No. 4,629,314 discloses a copying machine provided with a variable magnification operation memory function.
Japanese Patent 60-76769 discloses a means for selecting and setting a magnification using stored numerical data in a RAM which is backed up by the battery of a microcomputer.
According to the present invention, there is provided a variable magnification optical system for a copying machine, comprising a lens for projecting an image of an original document with a pre-selected magnification onto an imaging member, wherein at least one parameter may be altered to achieve the pre-selected magnification, means for generating a reference signal indicative of the nominal pre-selected magnification, means for generating a second signal indicative of the prevailing value of said parameter, and means for comparing the second signal with said reference signal and for automatically altering said parameter until the second signal matches said reference signal, and means for adjusting the value of the reference signal subtantially to compensate for tolerances between the nominal magnification and the actual magnification.
The system in accordance with the invention has the advantage that compensation can be made for tolerances between the nominal magnification and the actual magnification so that when the operator selects a particular magnification the copier optical system may easily be adjusted (either by the operator himself or a service engineer) to ensure that the copy magnification closely matches the chosen value.
On the other hand, the system may be adjusted purposely to yield an actual magnification which deviates from the nominal magnification by a desired amount in order to cater to particular customer requirements. For example, the system may be adjusted to achieve an actual magnification which is less than, say 99% of, the nominal magnification to ensure that none of the image is lost for edge to edge originals throughout the magnification range. Alternatively, the actual magnification may be increased to, say, 101% of the nominal magnification to ensure that the copies are free of black borders.
Suitably, the adjustment is made by means of an offset value which is stored in a memory and applied to modify the reference signal at all magnifications. Preferably, this offset value is entered into the memory via an operator accessible control panel, specifically a keyboard. The offset value may be determined simply by entering an integer which may be multiplied by a constant factor also stored in a memory before it is finally stored (and applied) as the offset value.
In a scanning optical system, the above magnification correction applies in the scan direction only. However, in the cross-scan direction, the position of the lens may be altered from its home position corresponding to nominal unity magnification to a new position corresponding to actual unity magnification in order to compensate for magnification tolerances in this direction also. Again, this correction may be made by the application of an offset value which may be entered and stored in a memory in a similar manner as for the scan-direction correction and applied at all magnifications.
An optical system in accordance with the invention thus has the advantage that it enables independent adjustment of the nominal magnification in two orthogonal directions.
An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing in which:
FIG. 1 is a schematic representation of a variable magnification scanning optical system for a xerographic copier in accordance with the invention, and
FIG. 2 is a diagram illustrating the control for the scanning optical system in FIG. 1.